The rapid ability of pathogens to develop resistance to antibiotics is endangering the management of a multitude of serious infections.1-3 In alarming contrast to the increase of bacterial adaptation to currently marketed drugs, the discovery of novel classes of antimicrobials is on the decline.4 Although the majority of pharmaceutical efforts during the past six decades have focused on the synthetic enhancement of a limited set of unique core scaffolds, a more sustainable route to combat antibiotic resistance is the discovery of novel chemical structures possessing unique microbial targets.2,5 Iron acquisition mechanisms in particular may represent effective antimicrobial targets that present substantial hurdles to bacterial antibiotic resistance.6 Iron is required for growth and survival of bacteria but remains tightly regulated in the mammalian host. Many pathogenic Gram-positive and Gram-negative bacteria utilize virulence-associated siderophores to scavenge iron from this restricted environment and return it to the microbial cell.7 Although previous studies have corroborated siderophore biosynthetic enzymes as effective drug targets through the tailoring of established chemical scaffolds,8-11 no novel chemical structures have been identified.
In our efforts to identify new structural antibiotic classes with inhibitory activity against siderophore biosynthetic enzymes, the Gram-positive bacteria methicillin-resistant Staphylococcus aureus (MRSA) and Bacillus anthracis were selected as model systems. The “superbug” MRSA is a major public health concern, attributed to more than 18,000 deaths a year in the United States.2-12 In contrast, the spore-forming microorganism B. anthracis is the causative agent of anthrax. The ability of the bacterium to quickly achieve high concentrations within infected hosts makes it a serious bioterrorism threat, with mortality rates for inhalational infection historically reaching as high as 94%.13 
Both pathogens are strongly associated with antimicrobial resistance,14 and their siderophore biosynthetic pathways have been extensively characterized.15,16 The siderophores staphyloferrin B (2) of S. aureus17-22 and petrobactin (5) of B. anthracis23-30 in particular have been shown to be critical for survival in iron-limited environments.
The biosynthetic pathways for both siderophores involve a type A nonribosomal peptide synthetase independent siderophore (NIS) synthetase, SbnE (FIG. 1A) in staphyloferrin B and AsbA (FIG. 1B) in petrobactin. Type A NIS synthetases are a unique class of enzymes found within siderophore biosynthetic pathways of a number of pathogenic bacteria, including Shigella flexneri, Escherichia coli, and Salmonella typhimurium.31,32 These enzymes catalyze the condensation of citric acid with either a polyamine or amino alcohol substrate in an ATP-dependent reaction.31,32 Since type A NIS synthetases share similar catalytic mechanisms and substrate preferences, novel antibiotics against S. aureus or B. anthracis that could also serve as broad-spectrum antibiotics against other NIS synthetase-containing pathogens were investigated.